1. Field of the Invention
The present invention relates to an improved electron gun and, more particularly, to an improved gun configuration having reduced electrostatic gradients enabling higher operating voltage without breakdown.
2. Description of Related Art
It is well known in the art to utilize a linear beam device within a travelling wave tube (TWT), klystron, or other charged particle device. In a linear beam device, an electron beam originating from an electron gun is caused to propagate through a tunnel or drift tube generally containing an RF interaction structure. At the end of its travel, the electron beam is deposited within a collector or beam dump which effectively captures the spent electron beam. The beam must be focused by magnetic or electrostatic fields in the interaction structure of the device in order for it to be effectively transported from the electron gun to the collector without loss to the interaction structure.
In particular, a TWT is a broad-band, microwave tube which depends for its characteristics upon interaction between the electric field of a wave propagated along a wave guide and the electron beam travelling within the wave. In this tube, the electrons in the beam travel with velocities slightly greater than that of the wave, and, on the average, are slowed down by the field of the wave. Thus, the loss in kinetic energy of the electrons appears as an increased energy conveyed by the field to the wave. The TWT, therefore, may be used as an amplifier or an oscillator.
The electron gun which forms the electron beam typically comprises a cathode and an anode. The cathode includes an internal heater to raise the temperature of the cathode surface to a level sufficient for thermionic emission to occur. When the potential of the anode is positive with respect to the cathode, electrons are drawn from the cathode surface and move towards the anode. In space charged limited flow, beam current is determined by the strength of the electrostatic field at the cathode surface. The geometry of the cathode, anode, and a focusing electrode provide an electrostatic field shape which defines the flow pattern. The electronic flow passes through an opening in the anode, and into the TWT. An electron gun of this type is known as a Pierce gun.
It has long been desired to increase the beam power of the typical Pierce gun, since a more powerful beam could result in more power being transferred to the wave. The operating voltage of the gun is roughly proportional to the beam output power, and increasing the operating voltage has been suggested as a method of increasing the beam power. However, if the operating voltage is increased beyond a threshold determined by the peak negative field gradient, the field becomes susceptible to breakdown. A breakdown condition is catastrophic to both the gun and the TWT. During a breakdown, a high voltage arc bridges between the anode and the cathode or the focusing electrode, further causing plasma generation which could ignite and destroy the gun and the TWT. For example, a Pierce gun operating at 600 kv would have a peak negative gradient at the focus electrode of approximately 200 kv/cm. Although this design might be sufficient for short pulse operation in the range of 1 .mu.sec, arcing would probably occur if the pulse length is extended to 5 .mu.secs and beyond.
One method of increasing the operating voltage of a Pierce gun entails partitioning the inter-electrode space with grading electrodes. This method has been described in R. True, "Design of Electron Sources and Beam Transport Systems for Very High Power Microwave Tubes," Proceedings of the Fifth National Conference on High Power Microwave Technology, United States Military Academy, West Point, N.Y., pp. 178-181, June 1990. In that paper, it was shown that with the use of grading electrodes along equipotential lines, the maximum voltage before breakdown increases substantially. Calculation of the maximum breakdown voltage in a Pierce gun is described in A. Staprans, "Electron Gun Breakdown," High Voltage Workshop, Monterey, Calif., February 1985, which provides the equation: EQU V=kL.sup.0.8
where L is equal to minimum inter-electrode spacing. Factor k is pulse-length dependent and is approximately equal to 9.times.10.sup.6, 6.times.10.sup.6, 4.times.10.sup.6, and 3.times.10.sup.6, for 1, 5, 100 .mu.sec pulses, and DC operation, respectively. For an inter-electrode space having n regions, the voltage breakdown for each region would be defined by the equation: EQU V'/n=k(L/n).sup.0.8
Therefore, V' would be equal to Vn.sup.0.2. In sum, the total breakdown voltage with the inter-electrode spacing partitioned into n regions is greater than the original breakdown voltage of a non-partitioned gun.
In a gun using three grading electrodes (n=4), the maximum voltage before breakdown would increase by a factor of 1.32. In high-power klystrons, peak output power is roughly proportional to PV.sup.2.5, where P equals perveance. For the three grading electrode example, maximum achievable power can be expected to double. Although this analysis neglects certain factors which can affect the high-voltage breakdown limit and the actual voltage and power increase may be less than double, it is nevertheless still very significant.
Nevertheless, high power applications continue to demand electron guns capable of producing increasing amounts of power. Thus, it would be desirable to provide a Pierce gun capable of producing increased beam power over that of a conventional gun using grading electrodes.